In the past it was difficult to determine the performance characteristics of an engine due to the fact that it was difficult to determine what was taking place within the combustion chamber of the engine. With the advent of ion sensing came the ability to determine the characteristics of combustion within a combustion chamber, allowing one to determine whether a fuel mixture was too rich or too lean and whether knocking or good combustion was taking place.
Ion sensing relies on the fact that combustion in an engine creates measurable ionized gas. In such an engine an ion sensor may be installed or, with proper circuitry, the ignition spark plug may be used to sense ion current without installing additional sensors. The ion sensor generates a small current that flows through the ionized gas in the combustion chamber, and amplifier circuitry is used to allow analysis of the combustion ion signal to diagnose engine performance characteristics.
Testing has identified that the combustion ion signal of a reciprocating engine, for instance, includes a first ion peak and a second ion peak. The first ion peak is due to the chemical ionization of the fuel and air in, or very near, the spark gap (if a spark plug is used) or the ion sensor. The second ion peak, or thermal peak, occurs after most of the fuel is burned and the remaining ion density near the sensor is approximately proportional to overall cylinder pressure.
Analysis of each of the ion peaks provides different combustion information. The second peak has been studied for some time and correlates with the peak cylinder pressure and indicates the location of the peak temperature, knock, and misfire, for example.
Difficulty has arisen, however, with regards to analysis of the first peak. Part of this difficulty is because the first peak can be 20 to 100 times the amplitude of the second peak. Prior ion amplifier circuits have been of a fixed, high gain type, optimized for amplifying the amplitude of the very small second peak. However, due to the high dynamic range between the two peaks, these prior ion amplifier circuits were unsuitable for analysis of the first peak. This is because the relatively large amplitude of the first peak compared to the second peak, such amplifiers often clipped this peak or became saturated, resulting in an unusable ion signal output for analysis.
There is a need in the art, therefore, for an amplifier circuit capable of interfacing with an ion sensor that is capable of producing a related output signal that can be used by a typical Electronic Control Unit (ECU) to analyze both the first ion peak and the second ion peak.
Embodiments of the present invention provide such a solution. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.